The ability of electromagnetic fields to extend to varying degrees through solid material makes possible the use of electromagnetic field generators and sensors for imaging of subterranean spaces. Instruments have been developed over time to take advantage of this ability of electromagnetic fields to penetrate solid objects. For instance, classic metal detectors generate an electromagnetic field and sense distortions in that electromagnetic field associated with varying conductivities and other characteristics of objects beneath or upon the surface of the ground or other objects.
Various different portions of the electromagnetic spectrum are useful for imaging different depths within soil. Modulating electromagnetic fields at various frequencies can achieve this multiple depth functionality. It is well known that low frequency electromagnetic signals can travel more deeply through solid structures, with at least a portion of the transmitted signal being detected at the surface and providing some indication of conditions deep within the soil. Such low frequency prospecting systems are used in oil exploration and other deep resource identification. High frequency signals (e.g. in the kHz range or higher) have the benefit of providing more specific information about subterranean objects. However, high frequency signals are readily absorbed by soil leaving little of the signal remaining for analysis. If objects or features to be found are too deep, such high frequency systems have proven generally ineffective. Thus, a significant need has existed for instruments which can provide a high degree of sensitivity to small changes in subterranean characteristics with enough of the signal surviving to be detected and return valuable information to the operator.
One technique which has shown some success is to transmit a frequency domain electromagnetic signal above and near the surface of the ground and sense a response to this signal in the form of a phase shift in the originally transmitted signal by a receiver located a fixed distance away from the transmitter. One such instrument is described in U.S. Pat. No. 5,654,637. While this instrument is effective in imaging underground anomalies it suffers from numerous drawbacks. The instrument is only portable in a very crude sense in that it can be picked up and hand carried. The practical survey range of such an instrument is therefore limited. Furthermore, its power levels are limited, such that only objects relatively close to the surface can be imaged with any degree of effectiveness. Also, such instruments are prone to saturation in high soil conductance environments and other high noise environments.
Increasing power to an electromagnetic signal transmitter is not merely a matter of increasing power to the transmitter. Rather, the signal generated by the transmitter must be carefully amplified in a manner which factors in details of the coil antenna and details of the associated transmitter circuitry. Furthermore, coil antennas are susceptible to self-capacitance, especially when operated at high power levels, such that the coil antenna increases in self-capacitance as the power level is increased and little or no real net benefit is realized by merely increasing the power to the transmitter antenna. Such difficulties, as well as high weight and bulk have kept the state of the art from advancing and limited the effectiveness of such subterranean imaging instruments.
The lack of portability and rates of data collection in prior art instruments have prevented effective utilization of subterranean imaging systems in the prior art for large subterranean area surveys. In addition, prior art instruments have a great tendency to saturate in high conductivity soil conditions and lack sufficient sensitivity in low conductivity soil conditions. Thus, small but significant values are not detected by such prior art instruments. Accordingly, a need exists for an instrument which can be made readily portable, ground mobile and gather data at a sufficiently fast rate to allow the instrument to be transported rapidly over the surface and gather highly detailed data, even for small details over a large area, as an area surveying tool.